How did scientists determine our location within the Milky Way galaxy—in other words, how do we know that our solar system is in the arm of a spiral galaxy, far from the galaxy’s center?
There is no short answer to this question, because astronomers have followed many lines of evidence to determine the location of the solar system in the Milky Way. But some of the general techniques can be outlined briefly.
Finding one’s location in a cloud of a hundred billion stars—when one can’t travel beyond one’s own planet—is like trying to map out the shape of a forest while tied to one of the trees. One gets a rough idea of the shape of the Milky Way galaxy by just looking around—a ragged, hazy band of light circles the sky. It is about 15 degrees wide, and stars are concentrated fairly evenly along the strip. That observation indicates that our Milky Way Galaxy is a flattened disk of stars, with us located somewhere near the plane of the disk. Were it not a flattened disk, it would look different. For instance, if it were a sphere of stars, we would see its glow all over the sky, not just in a narrow band. And if we were above or below the disk plane by a substantial amount, we would not see it split the sky in half—the glow of the Milky Way would be brighter on one side of the sky than on the other.
The position of the sun in the Milky Way can be further pinned down by measuring the distance to all the stars we can see. In the late 18th century, astronomer William Herschel tried to do this, concluding that the earth was in the center of a ‘grindstone’-shaped cloud of stars. But Herschel was not aware of the presence of small particles of interstellar dust, which obscure the light from the most distant stars in the Milky Way. We appeared to be in the center of the cloud because we could see no further in all directions. To a person tied to a tree in a foggy forest, it looks like the forest stretches equally away in all directions, wherever one is.
A major breakthrough in moving the earth from the center of the galaxy to a point about 3/5 away from the edge came in the early decades of this century, when Harlow Shapley measured the distance to the large clusters of stars called globular clusters. He found they were distributed in a spherical distribution about 100,000 light-years in diameter, centered on a location in the constellation Sagittarius. Shapley concluded (and other astronomers have since verified) that the center of the distribution of globular clusters is the center of the Milky Way as well, so our galaxy looks like a flat disk of stars embedded in a spherical cloud, or ‘halo,’ of globular clusters.
In the past 75 years, astronomers have refined this picture, using a variety of techniques of radio, optical, infrared and even x-ray astronomy, to fill in the details: the location of spiral arms, clouds of gas and dust, concentrations of molecules and so on. The essential modern picture is that our solar system is located on the inner edge of a spiral arm, about 25,000 light-years from the center of the galaxy, which is in the direction of the constellation of Sagittarius.
Credit: Laurence A. Marschall in the department of physics at Gettysburg College
NASA Scientists Study The Sun By Listening To It:
What’s the fastest way to understand space? According to NASA, it’s listening to the music of the spheres displayed as actual music. A program that converts astronomical data into sound is letting researchers blaze through years of data with ease. At NASA’s Goddard Space Flight Center, University of Michigan doctoral candidate Robert Alexander listens to audio files made from satellite data. The Wind spacecraft sits between Earth and the Sun, and records changes in the Sun’s magnetic field. Here’s how that becomes sound:When a person sings into a microphone, it detects changes in pressure and converts the pressure signals to changes in magnetic intensity in the form of an electrical signal. The electrical signals are stored on the reel tape. Magnetometers on the Wind satellite measure changes in magnetic field directly creating a similar kind of electrical signal. Alexander writes a computer program to translate this data to an audio file.
This mostly translates to white noise, but when there’s something anomalous, Alexander can hear it happen and make note of where in the file it happened. And Alexander isn’t the only one using data this way. In fact, he’s training other physicists who study the sun how to be active listeners. A similar project, onomatopoetically dubbed “PEEP,” wants to use sound as a monitoring tool, turning network activity into a gentle chorus of bird sounds, interrupted by frog croaks at the first sign of trouble.
Listen to a reverse shockwave headed toward the Sun below, and read more at NASA:
NYE Sunset - Flinders Street Station
Dream Idyll, a Valkyrie (Detail), 1902
Edward Robert Hughes (English, 1851-1914)